The present invention relates to a method for operating an internal combustion engine supercharged by way of an exhaust-driven turbocharger.
The performance of an internal combustion engine can be enhanced by supercharging, i.e. by precompressing the charge air before it enters the cylinders. In an exhaust-driven turbocharger, the compressor in the charge air duct is rotationally driven by a turbine that is arranged in the exhaust gas duct and is acted upon by the exhaust gas flow from the cylinder.
In order to enhance supercharging performance in particular in operating ranges of the internal combustion engine in which the exhaust gas volume is small and the basic torque of the exhaust-driven turbocharger is low, German Patent Application No. 32 44 928 A1 proposes diverting a partial flow of compressed charge air into the exhaust gas duct. The charge air is introduced together with the exhaust gas flow into the turbine, which is therefore accelerated more strongly with a higher air flow rate. Diversion of the charge air occurs in operating ranges of the internal combustion engine in which the supercharging pressure exceeds the exhaust gas pressure of the turbine. To initiate the diversion, a shutoff valve in an overflow duct is opened, and the partial flow of charge air that is to be diverted is driven to the turbine of the exhaust-driven turbocharger by the pressure drop in the overflow duct.
It is an object of the present invention to create a method for operating an internal combustion engine supercharged by way of an exhaust-driven turbocharger that optimizes supercharging performance over the entire operating characteristics field and decreases the exhaust emissions of the internal combustion engine.
The present invention provides a method for operating an internal combustion engine (1) supercharged by way of an exhaust-driven turbocharger (10), compressed charge air being diverted as a function of the operating point of the internal combustion engine (1), by opening a shutoff valve (8) in an overflow duct (7), out of a charge air duct (5) leading to the internal combustion engine (1) into an exhaust gas duct (6) of the internal combustion engine (1) to the turbine (11) of the exhaust-driven turbocharger (10) in the presence of a driving pressure drop (p2-p3) through the overflow duct (7), wherein when a pressure drop (p2-p3) driving toward the charge air duct (5) is present, the overflow duct (7) is enabled as necessary for the recirculation of exhaust gas into the charge air duct.
In a method according to the present invention, the overflow duct is used as an alternative to the diversion of charge air into the exhaust gas duct or to the recirculation of exhaust gas. With exhaust gas recirculation, exhaust emissions can be effectively decreased in many operating ranges of the internal combustion engine. Exhaust gas recirculation can be accomplished when a pressure drop driving toward the charge air duct exists in the overflow duct, i.e. when the exhaust gas pressure exceeds the supercharging pressure. If the internal combustion engine is to be operated with exhaust gas recirculation, the control valve in the overflow duct is opened, and the exhaust pressure drives a partial exhaust gas flow into the charge air duct. With the method according to the present invention, both charge air diversion and exhaust gas recirculation can be accomplished as required without additional structural complexity.
Advantageously, the flow rate of the overflow duct can be adjusted by varying the opening cross section of the shutoff valve, so that the increase in turbocharger torque (in diversion mode) and the exhaust gas recirculation rate (in exhaust gas recirculation mode) are adjustable. A control unit can ascertain the existing operating point on the basis of an input signal of at least one operating parameter of the internal combustion engine. The control unit then determines the operating mode associated with the particular operating point that has been ascertained (diversion mode or exhaust gas recirculation), and the corresponding flow rate of the overflow duct. The control unit activates the control valve in the overflow duct and establishes the predefined flow cross section.
The respective operating ranges in which diversion on the one hand, and exhaust gas recirculation on the other hand, are possible in accordance with the prevailing pressure drop can be enlarged if the pressure drop through the overflow duct is adjustable by way of a throttling element. Closing the throttling element backs up the exhaust gas flow and increases the exhaust gas pressure. With a corresponding increase in the exhaust gas pressure above the prevailing supercharging pressure, it is thereby possible for exhaust gas to be mixed into the charge air flow, if necessary, even at operating points at which the natural pressure drop prohibits any exhaust gas recirculation. By adjusting the throttling element correspondingly it is equally possible, in order to assist diversion, to establish a pressure drop through the overflow duct which drives the charge air. It is particularly advantageous to use, as the throttling element for the exhaust gas flow, an exhaust-driven turbocharger having a variably adjustable turbine geometry. The control unit can determine the exhaust gas pressure upstream from the turbine by correspondingly adjusting the flow cross section of the turbine, for example by adjusting a variable turbine guide baffle.